The invention relates to inline electron guns for cathode ray tubes and, more particularly, to an inline electron gun having a plurality of cathode assemblies, at least one of which is modified to have a turn-on time different from that of the other cathode assemblies.
A typical cathode-ray tube, e.g., a color television picture tube, includes an electron gun comprising three substantially identical cathode assemblies, each of which generates and focuses an electron beam which impinges on a different color phosphor element disposed on an interior surface of the tube faceplate. ln a conventional tube, the phosphor elements are red-, green-, and blue-emitting stripes or dots, which are stimulated to emission by the aforementioned electron gun assembly. The cathode assembly that provides an electron beam which impinges on the red-emitting phosphor element will be hereinafter referred to as the red gun. The same convention will also apply to the green gun and the blue gun.
In a television receiver, it is desirable that when the receiver is switched on, the picture appears quickly and with the proper colors. This capability is directly related to the rate of increase in electron gun cathode temperature. Some receiver manufacturers require that the initial screen color (with no video signal) be either white or green, rather than red, blue or purple. A white screen indicates equal color balance between the red, green and blue guns. A green color indicates that the cathode of the green gun has reached emission temperature before the cathodes of the red or blue guns.
In the prior art, it was known to preheat the cathode heater to meet the demand of quick turn-on. However, even a quick turn-on system does not always provide a white or green initial screen color. Furthermore, the preheated cathodes use power when the television receiver is out of use. Such a structure is energy inefficient.
A portion of a conventional inline electron gun such as that described in U.S. Pat. No. 3,772,554 issued to R. H. Hughes on Nov. 13, 1974 is shown in FIG. 1. The electron gun includes three substantially identical cathode assemblies and a plurality of spaced electrodes attached to a pair of glass support rods. Each of the cathode assemblies comprise a tubular cathode sleeve open at one end to accommodate a heater and closed at the other end by a cap having an electron emissive coating thereon. A cathode eyelet is attached to the open end of the cathode sleeve.
U.S. patent application Ser. No. 556,184, by R. E. Schlack filed on Nov. 29, 1983, assigned to the assignee of the present invention and incorporated by reference herein for the purpose of disclosure, describes an electron gun in which the emissivity of one or more of the cathode eyelets is modified to change the radiant emission characteristics thereof to cause the center cathode assembly (green gun) to reach emission temperature before the outer cathode assemblies (red and blue guns). In a test conducted using the eyelet structure described in the Schlack patent application, four of six tubes utilizing the modified eyelet structure showed initial green emission. The fact that two tubes did not was explained by uncontrolled changes in other parameters such as heater filament resistance, heater filament insertion variations in the cathode sleeve, unequal thermal changes or electron emissive material thickness variations of the cathode coating. A more reliable structure which provides initial green emission is desirable.